The £12m defences of the most heavily guarded power station in Britain have been breached by a single person who, under the eyes of CCTV cameras, climbed two three-metre (10ft) razor-wired, electrified security fences, walked into the station and crashed a giant 500MW turbine before leaving a calling card reading "no new coal". He walked out the same way and hopped back over the fence.

All power from the coal and oil-powered Kingsnorth station in Kent was halted for four hours, in which time it is thought the mystery saboteur's actions reduced UK climate change emissions by 2%. Enough electricity to power a city the size of Bristol was lost.

Yesterday the hunt was on for the man dubbed "climate man" or the "green Banksy". Climate activists responsible for hijacking coal trains and breaking on to runways said they knew nothing about the incident.

Even veterans of some of the most audacious direct actions, such as the scaling of the Kingsnorth chimney, are mystified. The station operator E.On professed astonishment that a lone activist would be daring enough to try to do something so potentially dangerous. Medway police said they had no suspects but were still investigating the incident, which took place on November 28.

"It was extremely odd indeed, quite creepy. We have never known anything like this at all, but it shows that if people want to do something badly enough they will find a way," said Emily Highmore, a spokeswoman for E.On.

Should "climate man" ever show up, he will be feted for what activists say was the most daring individual action of the year. "We have no idea who he is - but we really want to know. Everybody's asking 'where were you on Friday November 28'," said Ben Stewart of Greenpeace, one of six people arrested for climbing the 76 metre (250ft) chimney of the Kingsnorth station early last year but found not guilty of criminal damage in November. "We would never act anonymously," he added.

Yesterday the full story emerged of what happened. "It was about 10pm, very dark indeed," said Highmore. "It looks from the CCTV like he came in via a very remote part of the site by the sea wall and got over the double layer of fences."

The intruder then crossed a car park and walked to an unlocked door. But instead of going to the power station's main control room, where about eight people would have been working, he headed for its main turbine hall, where no one would have been working at that time.

Within minutes, says E.On, "he had tampered with some equipment" - believed to be a computer at a control panel - "and tripped unit 2, one of the station's giant 500MW turbines".

"This caused the unit to go offline," she added. "It was running at full 500MW load and the noise it would have made as it shut itself down is just incredible. CCTV shows that he then just walked out, and went back over the fence.

"It could be that no one has taken responsibility because they were so frightened by the noise it would have made. It's probably taken them a week just to get over the shock."

E.On, which wants to demolish the station and replace it with Britain's first new coal-fired power station in 34 years, said it was reviewing security, but doubted it was an inside job or the work of a big environment group. The intruder may have had some experience at one of Britain's other major power stations, insiders say.

"He left a banner but it was a real DIY job. It was really scrappy. This was an old bedsheet with writing done out of gaffer tape. It was very crude," said Highmore

"People at the station are gobsmacked," she added. "This is a different league to protesters chaining themselves to equipment. It's someone treating a power station as an adventure playground. You have to be trained to work here. People do not just wander about on their own. He could have killed himself. We do not have a problem with public protest but this was reckless. Whoever it was has crossed a line they should not have gone over. Power stations are dangerous places."

Kingsnorth was the site of a week-long activist camp in August which saw about 1,000 climate change activists try but fail to get into the station.

Notorious, but nameless

The Kingsnorth intruder joins a select group of "caped crusaders" who do their work without their names becoming widely known

Banksy: The graffiti artist whose work has attracted worldwide attention has taken his subversive style from urban Britain to the West Bank. He was recently unmasked by a Sunday paper, but after years of arresting images he has almost been elevated to status of national treasure.

Captain Gatso: The controversial leader of protest group Mad (Motorists Against Detection) has stoned, superglued, sprayed and ringed with burning tyres more than 1,000 roadside speed cameras in an eight-year campaign.

Superbarrio: Billed by his supporters as "faster than a speeding turtle and able to leap small speed bumps in a single bound", the flabby caped crusader in cherry red tights traverses the streets of Mexico City, defending the working class, the poor and the homeless. "I can't stop a plane or a train single-handed, but I can keep a family from being evicted," he said.

The Biotic Baking Brigade: A loosely connected group of leftwing activists, famous for throwing pies in the faces of such figures as the Microsoft's Bill Gates, the San Francisco mayors Willie Brown and Gavin Newsom and the Swedish King Carl Gustaf. The group's members have been active on animal rights and ecology issues as well as in feminist movements.